1. Field of the Invention
The present invention relates to a phase-shift digital holographic apparatus and a phase-shift digital hologram producing method. More particularly, the present invention relates to a phase-shift digital holographic apparatus in which an object wave and a reference wave fall simultaneously on an image forming surface of a photoelectric imaging device, such as a CCD image sensor, to produce holographic image data, and a phase-shift digital hologram producing method.
2. Description of the Related Art
Digital holography, in contrast to compute holography, records an interference pattern produced by interference between an object wave and a reference wave by a photoelectric imaging device, and the interference pattern is processed by a computer to reproduce an image. A known digital holographic apparatus projects an object beam and a reference beam on an image forming surface so that a very large angle .alpha. is formed between the object beam and the reference beam as shown in FIG. 2 to produce a hologram by the so-called off-axis system. A reproduced image includes a zero-order and .+-. first-order images in a lateral arrangement. Therefore, ghost images appear near the reproduced image and only a true reproduced image cannot be obtained. The object beam and the reference beam must be projected on the image forming surface so that a sufficiently large angle is formed between the object beam and the reference beam, and the degree of off-axis is large. An object which produces the object beam must be a small one and hence the conventional digital holographic apparatus is capable of forming holograms of only objects of limited sizes. Furthermore, the number of effective image pickup elements available for producing a hologram among those arranged in the image forming surface is reduced, and all the image pickup elements in the image forming surface cannot be effectively use for producing a hologram.
The conventional off-axis system projects the object beam and the reference beam on the image forming surface so that a large angle .alpha. is formed between the object beam and the reference beam. Therefore, intervals between fringes forming a hologram is small, and the resolution of the image forming surface must be smaller than the intervals between the fringes forming the hologram. Consequently, it has been difficult to reproduce an image satisfactorily by the image pickup device having a resolution lower than that of photographic films.